1. Field of the Invention
This invention relates to a catalyst for purification of diesel engine exhaust gas. More particularly, it relates to a catalyst which can purify the diesel engine exhaust gas of such harmful components as carbonaceous particulates, unburned hydrocarbons, and carbon monoxide by combustion from a low temperature upward and moreover suppress the formation of sulfate from sulfur dioxide.
2. Description of the Prior Art
In recent years, the microfine substances (mainly comprising solid carbonaceous particulates, sulfur type particulates such as of sulfates, and liquid or solid polymeric hydrocarbon particulates; hereinafter referred to collectively as "microfine substances") particularly in the diesel engine exhaust gas have been posing a problem from the viewpoint of environmental hygiene. The reason for this problem is that virtually all these microfine substances have diameters of not more than 1 .mu.m and, therefore, are liable to be suspended in the open air and taken into human bodies by respiration. Thus, studies are now under way in the direction of increasingly exacting the regulations concerning the discharge of such microfine substances from diesel engines.
Meanwhile, the amounts of microfine substances discharged from diesel engines have been decreased to some extent in consequence of such improvements as increased pressures for the injection of fuel into the diesel engines and enhanced control of the timing of fuel injection. These decreases of amount are still short of being fully satisfactory. Soluble organic fraction (SOF) which are contained in the microfine substances are formed mainly of liquid polymeric hydrocarbons. And they cannot be removed by such improvements in the diesel engine as mentioned above. As a result, the ratio of the SOF to the whole microfine substances has increased. Since the SOF contain such harmful components as cancerogenic substances, the removal of the SOF has become as important a problem as that of the microfine substances.
As a means for removing the microfine substances from diesel engine exhaust gas, the feasibility of the catalytic method which effects the removal of the carbonaceous particulates by seizing the microfine substances suspended in the diesel engine exhaust gas by the use of a catalyst obtained by depositing a catalytic substance capable of burning carbonaceous particulates on such a refractory three-dimensional structure as, for example, a ceramic foam, a wire mesh, a metallic foam, a pore-sealed type ceramic honeycomb, an open flow type ceramic honeycomb, or a metallic honeycomb and under the conditions of gas discharge (as to the composition and the temperature of gas) obtainable under the normal travelling conditions of a diesel engine or by the use of an electric heater or other similar heating means is now under study.
Generally, the catalyst for the purification of the diesel engine exhaust gas is desired to possess the qualities of (A) permitting highly efficient removal of harmful components including unburned hydrocarbons and carbon monoxide in addition to carbonaceous particulates from a low temperature upward, (B) avoiding obvious oxidation of sulfur dioxide (SO.sub.2) arising from sulfur components copiously contained in gas oil as the fuel into sulfur trioxide (SO.sub.3) and suppressing the formation of sulfates (such products of oxidation of sulfur dioxide as sulfur trioxide and sulfur mist), and (C) tolerating continuous service under high load, namely manifesting the so-called high durability to resist high temperature.
JP-A-61-129,030, JP-61-149,222, and JP-61-146,314, for example, disclose a catalytic composition having palladium and rhodium as main active components and additionally incorporating alkali metals, alkaline earth metals, copper, lanthanum, zinc, manganese, etc. EP-A-0092023 discloses a catalytic composition combining at least one member selected from the group consisting of copper, alkali metals, molybdenum, and vanadium with at least one member selected from the group consisting of platinum, rhodium, and palladium.
Further, a noble metal oxide catalyst of the shape of an open honeycomb furnished with through holes parallel to the flow of gas for use in the removal of SOF in the diesel engine exhaust gas has been reported (SAE Paper, 810263).
The conventional catalysts mentioned above are invariably effective to a certain extent in removing carbonaceous particulates by combustion or removing SOF. Of these conventional catalysts, those which contain platinum group metals manifest a great ability to oxidize sulfur dioxide under the conditions of discharge of gas at high temperatures and form sulfates in a notably large amount and, therefore, are at a disadvantage in entailing an environmental problem anew because of these sulfates.
A catalyst which is fully endowed with the aforementioned qualities of (A) to (C) required for the catalytic purification of the diesel engine exhaust gas and further vested with an ample ability to remove SOF has not yet been developed.
An object of this invention is to provide a catalyst for purification of diesel engine exhaust gas which permits highly efficient removal of microfine substances in the diesel engine exhaust gas.
A further object of this invention is to provide a catalyst for purification of diesel engine exhaust gas which possesses an ability to remove such harmful components as unburned hydrocarbons and carbon monoxide in addition to carbonaceous particulates in the diesel engine exhaust gas by combustion from a low temperature upward and, moreover, avoids obvious oxidation of sulfur dioxide and suppresses the formation of sulfates.
Another object of this invention is to provide a catalyst for purification of diesel engine exhaust gas which permits highly efficient removal of SOF in the diesel engine exhaust gas.
Yet another object of this invention is to provide a catalyst for purification of diesel engine exhaust gas which manifests an ideal ability to resist high temperatures and operates effectively as mounted on a diesel car without entailing any practical problem.